The word 'junk' means something which is similar to garbage or considered as garbage and it is of no use and very less in value. So we understand that junk food is a food which is of no use, causes diseases, is of very less value, kills people. A junk is very very bad for our health, it causes many diseases to us, harms us, sometimes even kill us. It is very very low in nutrients but high in calories and fats. Every junk food harms us. Therefore, we should never consume junk foods and consume only good foods that are good for health!! Some examples of junk foods are burger, pizza, chowmin, chips, soft drinks, etc!!
Brain is a very delicate organ of our body which is inside our head. It is a organ which controls the whole functioning of our body. It is connected with all the nerves of our body. If something is sensed by by our body the nerves carry signals to the brain and the brain responds to the signals and we understand what we have to do!!
I hope this answer might be helpful!!